Many advances have recently been made in the field of providing accurately synchronised time signals at dispersed locations. One particularly significant development has been the deployment of GNSS satellite navigation systems such as GLONASS and GPS. Other similar systems could be employed, where available. For brevity, references herein to “GPS” should be understood to include all such similar satellite navigation systems, including those employing so-called “pseudolites”, that is to say, ground-based transmitters which emit signals similar to those emitted by satellites of a satellite navigation system, and whose signals are interpreted by GPS receivers as if they came from a satellite of the GPS system.
Timing devices commonly known as GPS-Disciplined oscillators, or even GPSDOs, are well known. For example, U.S. Pat. No. 5,757,786 discusses an example, as do various reports of the UK's National Physical Laboratory, for example the articles by J. Davis and J. M Furlong in the 8th International Conference on Electromagnetic Measurement, 4–6th Nov. 1997, p. 11–1; NPL Report No CTM 1, October 1997, 11th European Frequency and Time Forum Neuchatel, 4–6th Mar. 1997, p. 515–520; and Proceedings 13th European Frequency and Time Forum, 13–16 Apr. 1999, Besancon, p. 291–295. These and other relevant publications are listed at www.npl.co.uk/time/public.html.
The time signals sent by GPS satellites are very stable, over a relatively long period of time. Each GPS satellite carries a very stable frequency reference, such as a Caesium atomic clock. However, various factors such as path length, multiple paths and atmospheric conditions cause the time signal as received at a terrestrial receiver to show apparent short-term drift. The GPSDO addresses this problem by providing a local oscillator which is very stable in the short term, but may show some drift over a longer term. This clock is compared to a received GPS time signal at regular intervals, and adjustments are made to the local oscillator signal to bring it into synchronization with the GPS time signal. This is known as ‘disciplining the oscillator to the GPS signal’, and the oscillator is said to be “GPS-disciplined”. Since the GPS signal is very accurate over a relatively long time scale, this disciplining prevents any drift in the local oscillator, while the local oscillator provides an accurate and stable timing signal which is free-running between GPS disciplining actions.
Such a GPS-disciplined oscillator can provide sub-microsecond accuracy, typically to the order of ±100 ns. Two independent GPSDOs placed at mutually distant locations could be expected to provide timing signals which differ only by around 100 ns. Such accuracy could otherwise only be produced by use of a very accurate, and hence very expensive, local oscillator.
A known GPS-disciplined frequency reference, such as the FLUKE 910/910R, is intended to produce very stable output frequencies along with an indication of real time (GPS Time), and optionally also of geographical position (GPS Position). Typically, such devices provide a time accuracy of 100 ns. Some devices are capable of an accuracy of 20 ns, but only under limited environmental conditions. The stability of the frequency and time outputs is derived from a combination of a stable clock, such as a Caesium atomic clock, carried on each GPS satellite, with a stable internal oscillator such as an oven controlled crystal oscillator or a rubidium standard which is disciplined to incoming GPS signals representing the GPS satellite's atomic clock. Such equipment typically provides a one-pulse-per-second (1 PPS) output. This 1 PPS signal is used to discipline the internal oscillator, which in turn produces at least one stable output frequency (Freq). These stable output frequencies are typically 10 MHz or 5 MHz, but other frequencies could be provided if required.
In present high-precision timing applications such as telecommunications and high accuracy multi-lateration, it is often required to provide synchronised timing signals at remote locations to within ±1 ns. Such accuracy is not possible with the GPSDO alone.
International Patent Application WO 01/61426 describes a method and apparatus used to address this problem. In that document, there is proposed a system having multiple GPS receivers at mutually distant locations. Each of these GPS receivers is connected to a central processor system. The central processor system receives timing signals from each of the GPS receivers. The central processing system then calculates the offsets between the various GPS receiver time signals, and stores values for these offsets. When one wishes to make use of the timing signal from a particular GPS receiver, the central processing system applies its calculated timing offset to the timing signal received from the GPS receiver in question, and supplies the resulting corrected timing as the output of the GPS receiver. The system described does not cause the various GPS receivers to be synchronised—“disciplined”—together, but simply tracks the timing offsets of each receiver. The system is also relatively cumbersome, in that the system needs to be provided with a central processing unit, each of the several GPS receivers needs to be connected to a central processing unit, and each request for time information must be made through the central processing system.
International patent application WO 99/63358 discloses a system of networked GPS receivers. The receivers communicate in order to generate a location estimate of increased accuracy. All of the GPS receivers are synchronised to GPS time. However, no attempt is made to improve the accuracy of synchronisation beyond the 20–100 ns accuracy which is normally produced by such arrangements.